虽然孩子已经是美国公民，但是申请Certificate of Citizenship的时候需要宣誓是法律规定的。
“[...] Upon proof to the satisfaction of the Attorney General that the applicant is a citizen, and that the applicant's alleged citizenship was derived as claimed, or acquired, as the case may be, and upon taking and subscribing before a member of the Service within the United States to the oath of allegiance required by this chapter of an applicant for naturalization, such individual shall be furnished by the Attorney General with a certificate of citizenship, but only if such individual is at the time within the United States.”
